In complex mechanical watches, there exist mainly two well-known forms of perpetual calendar mechanism. One may readily recognize the first of these mechanisms by a small dial bearing the names of the months for four years and which is swept by a month hand. The hand thus makes one revolution in four years. This arrangement permits firstly to recognize that there is a perpetual calendar and thereafter to know immediately to which year the watch has been set relative to the leap-year. The second mechanism corresponds to the general definition given hereinabove where the month indicator makes one revolution per year. In this case however there is nothing to indicate that one is concerned with a perpetual calendar watch and furthermore it is not possible to know off-hand whether the watch is correctly set relative to the leap-year if it is not provided with an additional indicator of the four year cycle.
Such an additional indicator may be found for example in the movement bearing number 861 389 of the manufacturer Patek-Philippe. In this construction the month star bearing the month hand and making one revolution per year controls a reducing gear train which drives a year hand to effect one revolution in four years, the axis of the year hand being coaxial to the month hand. It will be understood however that this system is complicated and above all is likely to require considerable space in the thickness dimension.